I have been regularly using the backup function of Windows Backup and Restore (Windows 10). My PC recently crashed. I have all my data restored but I cannot find any of my *.splsm or *.splsm.meta files.

It depends on where you have chosen to set your workspace directory for SmartPLS and whether this folder was included in your backup. It seems that this is more a Windows 10 Backup question rather than a SmartPLS problem.